This invention relates to methods and means of reducing noise and undesired signals in a radio receiver and more particularly for reducing an interfering signal among the Loran beacon signals received by a Loran receiver, by cancelling the interfering signal.
One type of interference in a Loran-C receiver takes the form of one or more narrow band signals at the edge of the band of received beacon signals (90 to 110 KHz). These interfering signals may come from other beacons that transmit just outside of the Loran-C band and, although they are outside the band, they are relatively very high peak power and will interfere with the reception of the desired beacon signals. Heretofore, some efforts have been made to reject these narrow band undesired signals by providing a reject filter in the path of the received signals in the Loran receiver, that includes a manually variable capacitor controlled by the Loran receiver operator. Some systems of this sort require the continual attention of a human operator, and are not automatic.
Automatic interference rejection in a Loran-C receiver is described in U.S. Pat. No. 4,549,312 entitled: A Radio Receiver with Automatic Interference and Distortion Compensation, filed Feb. 29, 1980, by Sheldon B. Michaels and the applicant herein. That patent describes a Loran-C receiver wherein beacon signals in the band 90 to 110 KHz are received and fed through several variable low and high narrow band, reject filters that reject the undesired narrow band interfering signals at the edges of the band of received beacon signals during an automatic reject mode. The reject filters are set during an automatic search mode using variable narrow band pass filters that are varied to sweep the low and high edges of the Loran-C band and the sweep control signals for these pass filters when each produces a maximum output is stored. Then, during the cancel mode, the variable low and high reject filters are tuned by these stored control signals. Thus, the variable reject filters are controlled by a feedback system that includes additional tuned filters (the pass filters) to produce control signals for the variable reject filters.